This invention relates to the detection of counterfeit objects. The invention will be described primarily in the context of the detection of counterfeit banknotes, but all aspects of the invention are applicable also to other documents, such as passports, cheques and trading stamps.
The production of counterfeit banknotes is continually increasing as a result of continuing improvements in printing technology, particularly colour printing. Counterfeit notes are now being made which appear, to the unaided eye, virtually indistinguishable from a genuine note.
It would be desirable to provide a device to assist a person conducting cash transactions, such as a shop assistant or bank teller, in verifying the authenticity of a received banknote. Such devices exist, but they rely for their operation on the experience and judgement of the user, and in any event are not very reliable. There are banknote discriminators which make numerous precise measurements to determine both authenticity and denomination of a banknote, but although these can be reliable, they are expensive, bulky and not suitable for use in, e.g., a shop where a customer's banknotes would have to be fully inserted into the machine before verification. The present invention seeks to provide a device which solves these problems, preferably by providing apparatus which can reliably be used to verify authenticity of banknotes held by a user without requiring accurate positioning and measuring techniques. However, the invention is applicable also to automatic discrimination devices, such as those in which the notes are fully inserted into a machine to enable measurements to be made.
Genuine monetary notes are now generally made to a specific formulation such as security or unbleached paper. Counterfeit notes, on the other hand, are generally but not always made from bleached paper. It is known to differentiate bleached from unbleached paper by viewing the paper under a source of ultraviolet radiation, such as an ultraviolet (UV) lamp which emits light having a wavelength which peaks in a band of from 300 to 400 nm.
Bleached paper includes chemical components which fluoresce when exposed to ultraviolet radiation; that is, the molecules in the composition of the paper are excited and emit light at a longer wavelength which peaks in the band of from 400 to 500 nm. Because wavelengths of 300 to 400 nm generally lie outside the spectral region of the human eye and because wavelengths of from 400 to 500 nm lie within the spectral region, the phenomena of fluorescence allows some counterfeits to be detected with the human eye.
This process can be automated with the use of electronics by providing a sensor and a comparator which compares the intensity of the fluorescent light sensed with a reference level so as to provide an indication as to whether the paper is a likely counterfeit or not. Such an apparatus is disclosed in U.S. Pat. No. 4,558,224. However, some genuine money notes if washed acquire a deposit of chemicals which fluoresce and some counterfeit notes are made with paper containing little or no fluorescent materials and so the fluorescing phenomenon is not always an infallible way of deciding whether a note is counterfeit or not.